This highlighted bit of code used to be an "else" statement instead of "else if", causing the client to basically send a packet every tick to the server. This change makes Reach, Autoclicker & other serverside anticheat checks more difficult, because you do not know when the client ticks.

Presumably this change has been made to reduce bandwidth usage, but the change should be insignificant: about 1 MB/hour/player saved by this change.
4,608,000 bytes, calculated by 64 byte minimum TCP packet * 20 ticks/second * 60 seconds/minute * 60 minutes/hour

It'd be great to return to this behaviour like in Minecraft 1.8 to make serverside anticheats more reliable. This is also already typical behaviour in Bedrock with server authoritative movement and is also used for serverside anticheats there.